Deep fat fryer with improved temperature control

ABSTRACT

A deep fat fryer has a control system ( 11 ) connected to a temperature sensor circuit ( 6 ) and to a heater control ( 13 ) for thermostatically activating a heating element ( 2 ) in response to a temperature signal from a temperature sensor circuit ( 6 ) representing a temperature at or below a lower limit value and deactivating the heating element ( 2 ) in response to a temperature signal representing a temperature at or above an upper limit value. The control system ( 11 ) is further adapted for, if the heating element ( 2 ) is active, generating the food lowering command signal in response to a temperature signal from the temperature sensor circuit ( 6 ) representing a predetermined sensed temperature below the upper limit value.

BACKGROUND OF THE INVENTION

The present invention relates to a deep fat fryer according to theintroductory portion of claim 1.

Such a deep fat fryer is known from U.S. Pat. No. 6,138,552. In thisdeep fat fryer, the lowering command signal is generated when theselected cooking temperature is reached by the cooking bath and causes abasket lift motor to immerse a basket carrying food to be fried into thefat in the frying pan.

A problem of this and other thermostatically controlled deep fat fryersis that the temperature of the fat sinks for a substantial period oftime after a batch of food has been lowered therein. This period of timevaries substantially as does the required cooking time for a givenquantity of a given food. The thermostatic temperature control willrespond to the temperature drop and activate the heating element.However, there is a lag between the time when the food is lowered in thefat causing an immediate temperature drop and the time when thethermostat calls for heat. Subsequently, it also takes some time beforethe temperature gradient associated to the maximum heat flow from theelement to the fat is established. The time lag between the temperaturedrop and the establishment of the maximum heat flow is normally 45seconds or more.

The temperature drop has an adverse effect on the fried food quality.The longer the duration of the temperature drop, the more fat isabsorbed by the food and the more water is lost from the food.

Another problem of deep fat fryers is that degeneration of the cookingmedium is accelerated and energy consumption is increased due toovershoot in the temperature of the cooking medium, which occurs in use.

In U.S. Pat. No. 3,894,483, it is described to overcome the thermostattime reaction lag by initiating burner operation before or as the foodis lowered into the cooking medium by either combining a bypass resettimer with a well-known adjustable frying cycle timer or manual startswitch, or by tying in the bypass reset and cycle timers with anautomatic lowering basket lift, such as that disclosed in U.S. Pat. No.3,273,488. However, the activation of the heater results in atemperature overshoot if the food to be fried is lowered into thecooking medium too late or if no food has been loaded into the basket.Conversely, if the food is lowered into the cooking medium at the sametime as heater operation is initiated or very briefly thereafter, heatflow to the cooking medium may still be building up after thetemperature of the cooking medium has dropped or the temperature of thecooking medium, which varies in accordance with the thermostaticallycontrolled activation and deactivation of the heater, may be relativelylow at the time of lowering of the food.

Also in U.S. Pat. No. 5,596,514 it is described to provide an“instant-on” feature that causes the heating element to be turned onimmediately, regardless of the cooking medium temperature. This featureis activated once a product key is depressed to initiate the cook mode.After a period of 15 seconds, a controller evaluates whether thetemperature has gone up or fallen. If the temperature has gone up, theheating element is turned off. If the temperature has fallen, theheating element remains on. Also this solution entails that theactivation of the heater can easily result in a temperature overshoot ifthe food to be fried is lowered into the cooking medium too late(depending on the temperature and the heating condition when the productkey is depressed) or if no food is lowered into the cooking medium atall. The food can also be lowered into the cooking medium too early,i.e. before the heat flow to the fat is fully established and/or whilethe temperature is near the low end of the temperature range maintainedby the thermostatic heater control.

SUMMARY OF THE INVENTION

It is an object of this invention to overcome the reaction lag betweenthe temperature drop associated with the lowering of food in the cookingmedium and the establishment of the maximum heat flow from the heater tothe cooking medium while avoiding or at least reducing the risk oftemperature overshoot due to food not being lowered into the cookingmedium in good time.

It is another object of this invention to save on the amount of energyrequired in a given cooking operation.

These objects are achieved by providing a deep fat fryer according toclaim 1. The generation of a food lowering command signal commanding thelowering of food to the cooking medium in response to a temperaturesignal representing a sensed temperature below the upper limit value ofthe cooking medium temperature at which the heater is deactivated andwhile the heating element is active supports optimal timing of thelowering of the food into the cooking medium, so that the temperaturedrop is minimized while the risk of temperature overshoot is avoided orat least reduced.

Particular embodiments of the invention are set forth in the dependentclaims.

Other objects, features and effects as well as details of this inventionappear from the detailed description of a preferred form of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation in side view of a deep fat fryeraccording to the present invention;

FIG. 2 is a graph of temperature over time of a first example ofoperation of a deep fat fryer according to the present invention; and

FIG. 3 is a graph of temperature over time of a second example ofoperation of a deep fat fryer according to the present invention.

DETAILED DESCRIPTION

In FIG. 1 an example of a deep fat fryer according to the invention isshown which includes a removable frying pan 1 and a heating element 2for heating a cooking medium 3 (usually frying fat) in the frying pan 1.To lower food into the cooking medium 3 and to lift food out of thecooking medium 3 a basket 4 and an automated basket lift 5 are provided.

A temperature sensor circuit 6 is provided for sensing the temperatureof the cooking medium 3 in the frying pan 1 and for generating atemperature signal representing the sensed temperature in the frying pan1. The temperature sensor circuit 6 includes a negative temperaturecoefficient probe 7 as is well known in the art.

A sound generator 9 in the form of a buzzer connected to the controlsystem 11 is provided. The control system 11 is adapted for outputting afood lowering command signal to the buzzer 9, which causes the buzzer 9to generate an audible food lowering command signal warning the userthat the food should be lowered into the basket 4 in preparation of thelowering into the fat 3. Alternatively, or additionally, also othersound generators, such as beepers and loudspeakers connected tocircuitry for generating signals driving the loudspeaker can beprovided. Furthermore, the fryer is equipped with a display 10 that maybe used to show optical command signals and other information, such asthe temperature setting. The control system 11 is further adapted forgenerating further food lowering command signals, controlling theautomated basket lift 5 to further lower the food into the fat 3.

The heating element 2 is included in a heating circuit 12 which includesa heating element control switch 13 for switching the heating element 2on and off to control of the temperature of the cooking medium 3 byactivating and deactivating the heater 2. The heating circuit 12 furtherincludes a number of safety features in the form of a pan presencesensor 18, a thermostatic switch 14 that opens in response to atemperature exceeding the maximum allowable frying temperature andautomatically closes when the temperature is lowered again to a levelunder the maximum allowable frying temperature and a safety fuse 15adapted to open in response to a highest safe temperature above themaximum allowable frying temperature.

The temperature sensor circuit 6 and the control switch 13 areoperatively connected to the electronic control system 1 1, which isadapted for thermostatically controlling the switch 13 for activatingthe heating element 2 in response to a temperature signal from thetemperature sensor circuit 6 representing a sensed temperature at orbelow a lower limit value and deactivating the heating element 2 inresponse to a temperature signal from the temperature sensor circuit 6representing a sensed temperature at or above an upper limit value.

Thus, the control system 11 generates a food lowering command signalcommanding the lowering of food into the basket 4 in preparation of thelowering of food into the cooking medium 3 and for generating a foodlowering command signal causing the basket 4 carrying the food to belowered into the fat 3 in response to at least one temperature signalthat represents a given sensed temperature below the upper limit valueto which the control system 11 is set. The control system 11 is furtheradapted to only generate food lowering command signals if the heatingelement 2 is active.

A first example of the operation of the described fryer is describedwith reference to the graph shown in FIG. 2. The continuous line in thegraph in FIG. 2 represents the cooking medium temperature over timeafter the fryer is switched on and if food is immersed into the cookingmedium after approximately 10 min 30 s. The food immersion may becompletely or partially. The interrupted line shows the cooking mediumtemperature over time if no food is immersed into the cooking medium 3.

First, the temperature rises while the heating element 2 is active. Inthe present example, the control system 11 is set via a user interface16 in the form of a control knob to a set point temperature (upper limitvalue) of 190° C. at which the heating element 2 is turned off. In thisexample, the associated turn-on temperature (lower limit value) at andbelow which the heating element 2 is switched on is 186° C. A first foodlowering command signal—in the graph designated as “load signal”—isgenerated in response to a first occurrence after switching on of thefryer or after warming up of the cooking medium 3 from a temperaturebelow the frying range (for instance from a temperature of 100° C. orlower) of a temperature signal representing a sensed temperature 10° C.below the pre-set upper limit value (in this example 190° C.) at whichthe heater 2 is switched off during frying at the selected temperaturesetting. This “load signal” warns the user that—if not already loadedinto the basket 4—food should be placed in the basket 4 in preparationof the imminent lowering of the basket 4 into the cooking medium 3.

A second food lowering command signal—in the graph designated as“immersion”—is generated in response to a first occurrence afterswitching on of the fryer or after warming up of the cooking medium 3from a temperature below the frying range (for instance from atemperature of 100° C. or lower) of a temperature signal representing asensed temperature 5° C. below the upper limit value of a pre-settemperature (in this example 190° C.) at which the heater 2 is switchedoff during frying at the selected temperature setting. This “immersion”signal causes the basket lift 5 to lower the basket 4 carrying the foodinto the cooking medium 3.

Then, according to this example, at t=10 min 30 s, the immersion of thefood into the cooking medium 3 causes the temperature of the cookingmedium 3 to drop to approximately 155° C. Because the heat flow from theheating element 2 to the cooking medium 3 is already fully established,after the temperature drop, the temperature of the cooking medium 3virtually immediately starts to rise back to the pre-set temperature,without the normal delay between passage of the switch-on temperatureand establishment of the full heat flow. In FIG. 2, the duration of thisnormal delay between passage of the switch-on temperature andestablishment of the full heat flow is in the idle situation representedby the interrupted line appears from the time span designated as“delay”.

Because the signals to load the basket 4 and to lower the basket 4 aregiven in response to temperature signals representing a sensedtemperature lower than the maximum temperature at which the heater isactive at the selected temperature setting, the risk of temperatureovershoot is reduced, while it is at the same time ensured that the heatflow from the heater 2 to the cooking medium 3 is not interrupted beforethe temperature drop and needs to be built up again while and after thetemperature drop has occurred. Furthermore, it is ensured that thetemperature of the cooking medium 3 is not far below the upper limitvalue when the food is lowered into the cooking medium 3.

The control system 11 is adapted to cause to generate the signal to loadthe basket 4 and to generate the signal to lower the basket 4 inresponse to temperature signals representing sensed temperatures belowthe pre-set temperature only upon the first occurrence of the respectivetemperature signals. Thus, it is avoided that the signals to lower foodinto the basket 4 and to lower the basket 4 are repeated when thetemperatures of 180° C. and 185° C. are reached again while the food isbeing fried. This can for instance be achieved by deactivating thegenerations of the basket lowering command signals in response to thesesensed temperatures 120 seconds after the respective signals have beengenerated.

Because the temperature of the cooking medium 3 rises quickly after thedrop associated with the immersion of the food to be cooked, the amountof fat absorbed in the food is kept relatively low, drying out of thefood is limited and the formation of excessive crust thickness isavoided. Because the frying time is not prolonged due to thethermostatic response time, food can be cooked in small portions withouteach time having to go through the thermostatic response time. Cookingin smaller portions further reduces the temperature drop and furtherreduces the time required to reach the upper limit value of thetemperature of the cooking medium 3 after the temperature. drop.Accordingly, the quality of the food can be further improved by cookingin small portions without requiring much more time.

It is observed that the temperatures described in the present examplecan also be chosen differently. Also, the differences between the upperlimit value of the sensed temperature and the predetermined sensedtemperatures in response to which the food lowering command signals aregiven need not be fixed. It is for instance possible to determine thetemperature in response to which a food lowering command signal is givenfrom the steepness of the temperature rise over time and the desiredtime between the signal and the moment when the upper limit value of thesensed temperature would be reached if no food is lowered into thecooking medium. Then, the food lowering command signal can be given apredetermined (and optionally adjustable) amount of time before thepre-set upper limit value of the temperature would be reachedindependently of the steepness of the temperature rise during heating ofthe cooking medium.

In FIG. 3, a second example of operation of the described deep fat fryeris shown. According to this example, the user interface 16 has beenoperated briefly after t=9 min to bring the control system 11 in a boostcondition controlling the heater 2 to be active to a temporarilyincreased upper limit value of the cooking medium temperature (in thisexample 200° C.) above the pre-set upper limit value of the sensedtemperature (in this example 190° C.).

Approximately at t=10 min, the control system 11 generates the “loadsignal” activating the buzzer 9 in response to a sensed temperature 10°C. below the temporarily increased upper limit value of the cookingmedium temperature at which the heating element 2 is or would bedeactivated if no food is lowered into the cooking medium. Approximatelyat t=10 min 45 s, the food immersion command signal causing the basket 4to be lowered is generated in response to a temperature signalrepresenting a sensed temperature 4° C. below the temporarily increasedmaximum temperature at which the heater 2 is active when the controlsystem 11 is in boost condition at the selected temperature setting. Asin the previous example, it is ensured that the heat flow from theheating element 2 to the cooking medium 3 is already fully establishedand not switched off again when the food is immersed (partially orcompletely) in the cooking medium 3. Accordingly, after the temperaturedrop, the temperature virtually immediately starts to rise back to thepre-set temperature, without a delay due to the response time of thethermostat and the time required to establish the temperature gradientassociated to the maximum heat flow from the heater 2 to the cookingmedium 3.

Again, the food lowering command signals warning the user to load foodinto the basket 4 and controlling the lift 5 to lower the basket 4 aregiven before the heater deactivation temperature that is in force hasbeen reached, so that the risk of temperature overshoot is reduced.

It is observed that the temporary temperature increase caused by thetemporarily increased upper limit value at which the heater isdeactivated may indeed cause a slightly accelerated degradation of thecooking medium. However, this effect is small, especially if, as isintended, food is lowered into the cooking medium before the temporarilyincreased upper limit value of the cooking medium temperature isreached. Furthermore, a higher cooking medium temperature at the timefood is lowered into the cooking medium results in a quicker heattransfer to the food. This in turn results in a reduced fat absorptionand a shorter cooking time.

The temporary increase of the upper limit value of the cooking mediumtemperature at which the heater 2 is deactivated further allows toensure that the heater 2 is activated for a sufficiently long period oftime to establish full heat flow from the heater 2 to the cooking medium3 even if the cooking medium temperature is at its maximum within therange associated to the selected temperature setting when the boostcondition is activated. To reliably achieve this effect, the temporaryincrease of the upper limit value is preferably at least 5 to 10° C.

In fryers which are not equipped with an automatic basket lift or whichare not equipped with a powered basket lift at all so that the basketmust be lowered and lifted manually, the food lowering command signalmay for instance consist solely of a human perceptible signal(preferably a sound signal) indicating that it is time to lower thebasket carrying food into the cooking medium or of a first humanperceptible signal warning that the basket should be loaded and a secondhuman perceptible signal indicating that the basket should be lowered.

The increased upper limit value of the temperature at which the heater 2is deactivated when the control system 11 is in boost condition can bedetermined as a separate limit value for the signal representing thesensed temperature. However, it is also possible to determine thattemperature indirectly by setting a predetermined maximum duration ofthe boost condition which, taking into account the power of the heater 2and the thermal capacity of the cooking medium 3 and the pan 1, providesa predetermined approximate temperature rise if no food is lowered intothe cooking medium. This provides the advantage that a single controlparameter is sufficient to limit both the maximum temperature in theboost condition and the duration of the boost condition. Furthermore,the duration of activation of the heater 2 is ensured independently ofthe current cooking medium temperature at the time the boost conditionis initiated.

If the maximum temperature of the boost condition is controlled bysetting a temporarily increased limit value for the signal representingthe sensed temperature at which the heater 2 is deactivated, thisincreased limit value is preferably determined by adding a predeterminedincrease to the pre-set upper limit value of the sensed temperatureapplicable during frying. The maximum temperature achievable in theboost condition is then closely related to the pre-set fryingtemperature. The duration of the boost condition can then be limited ina simple manner by ending the boost condition in response to atemperature signal representing that increased upper limit value or inresponse to the expiry of a predetermined time-interval after initiationof the boost condition. Unnecessary repetition of the food loweringcommand signals after the food has been lowered into the cooking mediumcan be avoided by adapting the control system 11 to generate foodlowering command signals in response to predetermined temperaturesignals associated with the boost condition only if the control system11 is in boost condition. In addition, the control system 11 ispreferably adapted to end the boost condition early enough to avoid thatthe normal upper limit value of the cooking medium associated to thetemperature setting that is in force is reached again after thetemperature drop caused by the lowering of the food into the cookingmedium before the boost condition has ended.

In order to avoid unnecessary repetition of the food lowering commandsignal or signals, it can be provided that the temperatures in responseto which these signals are generated by the control system 11 are notreached when the control system 11 is not in boost condition and beforethe boost condition has ended after lowering of the food into thecooking medium 3. Alternatively, for the same purpose, it can beprovided that the food lowering command signal or signals are generatedby the control system 11 in response to temperatures or a temperatureassociated to the boost condition only if the control system 11 is inboost condition and that the boost condition is ended early enough toprevent that, after lowering of the food into the cooking medium 3, thetemperatures or temperature in response to which the food loweringcommand signals are generated if the control system is in the boostcondition are reached again before the boost condition has been ended.Yet another alternative solution to prevent unwanted repetition of foodlowering command signals is to adapt the control system 11 to generatethe food lowering command signal or signals only once in response to thepredetermined temperature or respective temperatures after eachactivation of the boost condition. The food lowering command signal orsignals may each include a plurality of successive stimuli, such assounds or light signals.

1. A deep fat fryer including: a frying pan; a heating element forheating a cooking medium in the frying pan; a temperature sensor circuitfor sensing the temperature of the cooking medium in the frying pan andgenerating a temperature signal representing the sensed temperature inthe frying pan; a heater control for activating and deactivating theheating element; and a control system operatively connected to thetemperature sensor circuit and to the heater control, the control systembeing adapted for thermostatically activating the heating element inresponse to a temperature signal from the temperature sensor circuitrepresenting a sensed temperature at or below a lower limit value anddeactivating the heating element in response to a temperature signalfrom the temperature sensor circuit representing a sensed temperature ator above an upper limit value; and for in response to a temperaturesignal from the temperature sensor circuit, generating a food loweringcommand signal commanding the lowering of food; characterized in thatthe control system is adapted for generating the food lowering commandsignal in response to a temperature signal from the temperature sensorcircuit representing a predetermined sensed temperature below said upperlimit value, on condition that the heating element is active.
 2. A deepfat fryer according to claim 1, wherein the control system is adaptedfor generating the food lowering command signal in response to a firstoccurrence of the temperature signal from the temperature sensor circuitrepresenting a predetermined sensed temperature below said upper limitvalue after switching on of the fryer or after heating up the cookingmedium from a temperature below a lowest possible frying temperature. 3.A deep fat fryer according to claim 1, further including a userinterface operatively connected to the control system for setting aboost condition in which boost condition said upper limit value of thesensed temperature and said predetermined sensed temperature below saidupper limit value are temporarily increased.
 4. A deep fat fryeraccording to claim 3, wherein said control system is adapted fordetermining said temporarily increased upper value of the sensedtemperature by adding a predetermined increase to said upper limit valueof the sensed temperature.
 5. A deep fat fryer according to claim 4,wherein the control system is adapted for ending the boost condition inresponse to a temperature signal representing said increased upper limitvalue.
 6. A deep fat fryer according to claim 3, wherein the controlsystem is adapted for ending the boost condition in response to expiryof a predetermined period of time after the start of the boostcondition.
 7. A deep fat fryer according claim 1, further comprising atleast one signal generator adapted for generating a human perceptiblefood lowering command signal in response to a food lowering commandsignal from the control system.
 8. A deep fat fryer according to claim7, further including a basket and a basket lift for lowering the basketinto the cooking medium in the frying pan and lifting the basket out ofthe cooking medium, and adapted to lower the basket into the cookingmedium in response to a food lowering command signal from the controlsystem, the control system being adapted to generate the food loweringcommand signal causing the generation of the human perceptible signalbefore the generation of the food lowering command signal causing thebasket lift to lower the basket into the cooking medium.